1. Field of the Invention
The present invention relates generally to plating baths and methods for monitoring the constituents contained therein. More particularly, the method of the present invention relates to a voltammetric analysis technique for monitoring the concentration levels of certain plating bath major constituents such as acid. The method can be used to maintain desired constituent concentrations in order to ensure optimal plating bath performance.
2. Description of Related Art
A typical plating bath solution is comprised of a combination of several distinct electrochemical constituents which can be broadly divided into major constituents and trace constituents. The major constituents typically make up about 2 to 50 percent of the total bath weight or volume. Trace constituents are present in smaller quantities, usually less than 1 percent of the total weight or volume. Acid is an important major constituent in many plating baths. For example, in an acid cadmium plating bath, acid typically represents about 5 to 10 percent of the total bath weight.
The concentration levels of both major and trace constituents will influence the quality of the resultant plating deposit, and should therefore be regularly monitored. Methods have been developed for in-tank monitoring of trace constituents as well as certain major constituents. For example, U.S. Pat. No. 4,631,116 discloses a method for monitoring trace constituents using an in-tank electrochemical sensor. Application Ser. No. 08/037,158 entitled "Method of Monitoring Major Constituents in Plating Baths" discloses a method for in-tank monitoring of major constituents such as sulfuric acid in an acid copper bath. The above patent and pending patent application are owned by the same assignee as the present invention. However, these techniques provide less than optimal accuracy for certain types of acid, particularly in plating baths in which large quantities of hydrogen are produced during plating. For example, the above voltammetric techniques are not well-suited to measurement of acid in an acid cadmium bath. As a result, alternative techniques are currently used to measure acid cadmium concentrations.
One such technique involves the use of two sensors; one voltammetric, the other a conductivity sensor. The two sensors are required because often voltammetric sensors measure non-acid constituents, whereas conductivity relates to acid concentration. Other techniques currently used to measure relatively high acid concentrations in plating baths which produce large amounts of hydrogen during plating include pH sensors which are not very accurate at high acid concentrations. Use of the current measurement techniques is inconvenient, time-consuming and costly, since these techniques are not directly compatible with the voltammetric trace and major constituent measurement methods discussed above. Additional tests must be performed using a different set of equipment in order to properly monitor certain acid concentrations. No integrated measurement system is available which is capable of measuring these acid concentrations as well as most other major and trace constituent concentrations.
As is apparent from the above, there presently is a need for an accurate and inexpensive real time method of monitoring acid concentrations in plating baths which produce large quantities of hydrogen. The method should require the use of only a single in-tank sensor. Furthermore, the method should complement and be easily integrated with known techniques and equipment suitable for measuring other plating bath constituents, resulting in an efficient overall plating bath analysis system.